This invention relates to helical screw rotary compressors and more particularly, to a multiple slide valve assembly for selectively varying the capacity of the compressor using a first slide valve, and for varying the internal volume ratio of the screw compressor by a second slide valve integrated to the first slide valve while maintaining machine operation at full load.
Helical screw rotary compressors have incorporated means for varying the internal volume ratio of the screw compressor (variable Vi) to match the system pressure ratio by varying the location of radial porting while still keeping the machine operating at full load. This requires the re-positioning, in a controlled fashion, of a Vi slide valve without unloading of the machine by shifting a further capacity control slide valve. U.S. Pat. Re 31,379, reissued Sept. 13, 1983 and assigned to the common assignee, is one example of such a compressor.
Other U.S. Pat. Nos. directed to such a variable Vi concept known to the applicants are 2,519,913; 3,088,658; 3,088,659; 3,314,597; 4,455,131; 4,457,681; 4,597,726 4,609,329; 4,678,406; 4,610,612; 4,610,613 and 4,678,406.
Of the patents. U.S. Pat. Nos. 4,678,406 and 4,609,329 are representative of such axial flow helical screw compressors with a capacity control slide valve and a second slide valve or slide stop (variable volume ratio control member) mounted in face communication with the intermeshing rotors. In U.S. Pat. No. 4,678,406, the volume ratio is changed by stepwise movement of the slide stop, while the slide valve is shifted in an infinitely variable fashion for changing compressor capacity during compressor operation, and wherein the positions of both are varied in response to system operating parameters. With separate control of the slide stop and the capacity control slide valve in the event of interference between the movements of these members, the components are so designed that the movement of the slide stop overpowers that of the slide valve.
While the approach taken by U.S. Pat. No. 4,678,406 is laudatory and separate, dual controls of compressor capacity and variable Vi is achieved, stepwise movement of the slide stop is limited to but several steps, and the displacement of the slide valve and slide stop is effected from opposite ends of the end abutted slide valve and slide stop.
In U.S. Pat. No. 4,609,329, the variable Vi and capacity control of the compressor is again achieved by an end abutting capacity control slide valve and a further variable Vi slide valve or slide stop having faces confronting the intermeshed helical screw rotors. However to insure a compact arrangement for separate actuation of the slide valve and slide stop positioned centrally beneath laterally intersecting bores bearing intermeshed helical screw male and female rotors, is a longitudinally extending cylindrical recess which communicates with both the inlet and outlet ports at opposite ends of the intersecting laterally spaced bores. Mounted for slidable movement in that recess is a compound valve member in the form of an end abutting capacity control slide valve and a cooperating longitudinally shiftable side stop. The variable positioning of the slide valve is effected via a rod which passes through a slide valve inner bore and connects to a head proximate to the discharge side of the machine. The slide valve rod passes through the inner bore of the slide stop, with the slide stop, at its end opposite that abutting the slide valve, terminating in a head which mounts a first reciprocating piston within a cylindrical chamber facilitating the longitudinal shifting of the slide stop. The slide valve rod projects through the first piston and through a stationary end wall or end cap. A second, reciprocating piston is mounted to the end of the slide valve rod and lies within a second chamber defined by a fixed cylindrical casing. The fixed cylindrical casing slidably mounts both the first piston fixed to the slide stop and the second piston fixed to the end of slide valve rod.
Under such conditions, the compressor permits a controlled variation in its volumetric capacity simultaneously with controlling its compression ratio. The slide valve and the slide stop may be controlled to match the internal compression ratio in the compressor to the system compression ratio as the volumetric capacity is controlled, and when the slide valve and slide stop ar.RTM.moved apart, the axial space therebetween communicates with the intermeshed helical screw rotors to permit working fluid in a closed compression chamber or closed thread between the rotors at inlet pressure to remain in communication with the inlet port through a longitudinal slot within the casing proximate to faces of the slide valve and the slide stop to decrease the volume of fluid which is compressed. Thus, maximum capacity is provided with the slide valve and slide stop in abutting relation. A suitable control system is provided for moving the slide valve and slide stop in accordance with a predetermined program by sensing a number of variables from the compressor system, which signals are fed to a microcomputer, and the microcomputer output provides the desired control of the slide valve and the slide stop via its internal program. In addition, appropriate readouts or displays are effected to indicate the positions of the slide valve and the slide stop.
While the compressor of U.S. Pat. No. 4,609,329 achieves compactness, permits instantaneous external readout of separate slide stop and slide valve positions, the fineness of control to match the internal volume ratio of the screw compressor to the system pressure ratio is not of the requisite degree desired, and the exact positioning of the variable Vi slide stop via the microprocessor is not insured.
It is therefore a primary object of the present invention to provide a helical screw rotary compressor whose internal volume ratio may be readily changed to exactly match the system pressure ratio, while keeping the compressor operating at full load, in which concentric hydraulic cylinders include an internal hydraulic cylinder bearing a longitudinally movable piston which is solidly attached to a large Vi slide valve and within which is internally, mounted a smaller capacity control slide valve the assembly and which facilitates movement as an assembly of the internal cylinder and the large slide valve, within an outer cylinder, in which movement of the assembly as a whole is controlled by a stepping member through a ball screw mounted on the internal cylinder, and in which a rotary motion converted to linear scale output indicates the internal volume ratio of the machine.